1. Field of the Invention
The present invention relates to a method for producing a magnetostrictive element, and a method for sintering an object to be sintered containing a hydride.
2. Description of the Related Art
Magnetostrictive elements have been used for various devices, e.g., linear actuators, vibrators, pressure torque sensors, vibration sensors and gyro sensors.
When a magnetostrictive element is used for a linear actuator, vibrator or the like, a driving force is generated by changing a magnetic field applied to change the dimensions of the magnetostrictive element. When a magnetostrictive element is used for a pressure torque sensor, vibration sensor, gyro sensor or the like, on the other hand, it detects its permeability changing with dimensional changes of the magnetostrictive element caused by external pressure and thereby sensing is conducted.
These magnetostrictive elements are produced by compacting an alloy powder of given composition into a compact in a magnetic field and sintering it in an inert gas atmosphere (See, for example, Japanese Patent Laid-Open No. 2003-3203, Page 4).
During the sintering step, a compact to be sintered into a magnetostrictive element tends to be oxidized, and also discolored by radiation heat from the heater as the heat source for sintering. Therefore, a compact is contained in a closed container.
However, the conventional techniques involve the following problems.
When starting material of a compact to be sintered into a magnetostrictive element contains a hydride, the hydride is thermally decomposed to generate hydrogen during the sintering step and hydrogen is released from the compact. If hydrogen is generated while a compact to be a magnetostrictive element is contained in a closed container, hydrogen is filled in the closed container to increase vapor pressure, finally preventing hydrogen generated in the compact from being released from the compact. Moreover, in the compact, a solid-state reaction of its microstructure starts from the surface as the sintering step proceeds, which further retards release of hydrogen from the compact.
As a result, the magnetostrictive element thus produced has nests formed by the gasified hydrogen bubbles, which deteriorate strength and magnetic properties of the magnetostrictive element, and also reduce yield. This trend is more noted as a size of the compact increases.